KDE4, xfce4, razor, lxde, openbox - no problems. Only in fluxbox.
krusader (version 2.3.0-beta1) sets motif-windowmanager-hints, short _MOTIF_WM_HINTS:
$> xprop _MOTIF_WM_HINTS <click_to_krusader>
as you can see, the property contains the following:
_MOTIF_WM_HINTS(_MOTIF_WM_HINTS) = 0x2, 0x3, 0x1e, 0x0, 0x0
the 5 fields are defined es:
4. input mode
the decoration field is set to 0x1e. that bitmask is the result of logical OR of one of the following values:
1 - decor all, 2 - border, 4 - handles, 8 - title, 16 - menu, 32 - iconify, 64 - maximize
0x1e looks a lot like decimal 30 to me, thus it is 2 + 4 + 8 + 16 == border + handles + title + menu. and iconify and maxmize are _missing_.
since fluxbox respects the motif-window-manager-hints your only option is to complaint about this issue @ krusader-upstream. they should either dont use _MOTIF_WM_HINTS (try 'xprop -remove _MOTIF_WM_HINTS <click_krusader>' and then restart fluxbox .. maximize / minimize button appear) or to include the appropriate flags.